The big picture: Community ecology
Inter-specific Competition (-/-)
- Individuals sharing space will compete for resources
- reduces fitness for each species
- Resources can mean many things:
- prey items, water, nutrients, space
- Competitive exclusion:
- local extinction
- rare in nature (sometimes on islands)

Natural selection favors those that do not compete
- Ecological niche:
- abiotic factors, food choice, nest sites
- competition occurs for these resources
- Natural selection reduces overlap between niches!
- resource partitioning: division of limited resources
- Resource division allows for coexistence
- use parts of a habitat, active at different times
- allows partial niche overlap (not full)
- why competitive exclusion is rare!

Resource partitioning in Anolis lizards

There is serious DRAMA in the animal kingdom!!!

- Review: all animals are heterotrophic….
- All animals must eat, which means that most organisms are also at the risk of being eaten
- Exploitation: species feeding interactions (+/-)
- predation, herbivory & parasitism
- the drama drives natural selection!
Predation (+/-)
- One species (predator) kills and eats other (prey)
- Lots of feeding adaptations in the animal world
- fangs, claws, echo-location, venom, etc. etc. etc.
- Fitness (reproductive success) depends on feeding

Predation (+/-)
- Not getting eaten is just as important as eating
- powerful driver of natural selection
- Behavioral adaptations
- hiding, fleeing, herding
- active self defense less common
- Morphological adaptations
- spines, odors, toxin accumulation
- mimicry and camouflage

Defenses vs Predation → Co-evolutionary arms race

Herbivory (+/-)
- Organism (herbivore) eats part of plant or alga
- Herbivores are large & small, terrestrial & aquatic
- Like predators, many special adaptations
- chemical sensors to identify toxins
- specialized teeth or digestive systems
- Review: Plants don’t move
- just as many defensive adaptations
- thorns, toxins, etc.

Parasitism (+/-)
- Organism (parasite) gets nourishment from host
- host harmed but rarely killed
- ~1/3 of all species are parasites!!
- Parasites live inside or outside body of host
- Parasites usually have complex life cycles
- feeding and reproductive cycles using host

Species interactions are not all bad!
- Positive interactions occur where one at least one species benefits and their other is not harmed
- Positive interactions common in ecological communities
- drive higher biodiversity
- Mutualism and commensalism are the most common

Mutualism (+/+)
- Occur when both species benefit from the interaction
- Often, the 2 species depend on each other for survival/reproduction
- plants + insects = pollination + food
- Usually still a cost, but benefit > costs
- carbon - nitrogen trade-off between plants and fungi

Commensalism (+/0)
- Commensalisms are interactions where one benefits but doesn’t harm/help the other
- Some biologists argue that any close interaction between two organisms is unlikely to be completely neutral for either party


Trophic (feeding) structure defined by species interactions
- Structure and dynamics of a community depend on feeding relationships
- Energy is transferred upward
- plants/aglae → consumers
- transfer is wasteful (most energy is lost)
- Trophic level:
- many species can share a trophic level

Trophic Structure
- Structure and dynamics of a community depend on feeding relationships
- Energy is transferred upward
- plants → consumers
- transfer is wasteful (WHY?)
- Exists in terrestrial and aquatic systems
- What happens if a species goes extinct?

Food webs are multi-dimensional


Some species in a community are more impactful than others